In a magnetic recording and reproducing apparatus, a bias current value which is most suitable for a magnetic characteristic of a magnetic tape used is required to be automatically set. In the prior art, variation in a reproduced output level is detected by sequentially changing a bias current from its low to high value or from its high to low value while a test signal is being recorded and reproduced, and then determining the optimum bias current value which provides the maximum reproduced output level.
Of late, there have been used various kinds of magnetic tapes which have the optimum bias current value different from each other. Although the optimum bias current value depends on a magnetic head used, for example, it is about 1.0 mA for a normal tape, about 2.0 mA for a chromium dioxide tape and about 4 mA for a metallic tape. Accordingly, in order to set optimum bias current value for these three kinds of magnetic tapes, the bias current is required to be sequentially changed over a wide range of 0 to 5 mA, for example. However, this causes consumption of time for setting the optimum bias current for the particular magnetic tape especially because the bias current value should be changed bit by bit for precisely setting optimum bias current value for overcoming such a problem. It may be proposed that the optimum bias current value is set in a shorter time by changing the initial value of bias current sequentially in accordance with the type of magnetic tape used in association with a manual operation of selecting means such as a tape selector switch. However, this requires a troublesome operation by the user and inevitably makes the construction of the circuit complicated. Furthermore, since the reproduced output level depending on the bias current value near the optimum bias current value is substantially equal, it is difficult to precisely determine the peak bias.